1) Field of the Invention
The invention relates to a process for the hydrogenation of chloromethanes of the formula CH.sub.n Cl.sub.m, where n=0 to 3 and m=4-n.
2) Description of the Related Art
In industrial processes for the manufacture of chlorofluoromethanes, the corresponding chloromethanes are predominantly used as educts, in which chlorine is then replaced with fluorine in stages using HF. CCl.sub.4, in particular, has hitherto been used as the starting compound for the manufacture of chlorofluorohydrocarbons (CFCs). Thus, starting from CCl.sub.4, the series of compounds CFCl.sub.3, CF.sub.2 Cl.sub.2 and CF.sub.3 Cl are obtained with increasingly stringent reaction conditions as regards the amount of HF used, the temperature, the pressure and the type and amount of catalyst.
The CCl.sub.4 used for the manufacture of CFCs originated predominantly, either as the main product or as a by-product, from the common synthetic processes for the manufacture of chloromethanes. These are the thermal chlorination or catalytic oxychlorination of methane, in which all 4 chloromethane derivatives are obtained together, the chlorolysis process (chlorinating scission of propene or chlorine-containing C.sub.1 to C.sub.3 residues) and the esterification of methanol with hydrochloric acid to give methyl chloride, followed by further chlorination of the methyl chloride to give more highly chlorinated chloromethanes.
In the strongly exothermic gas phase chlorination of methane (400.degree. -450.degree. C., slight positive pressure), which proceeds via free radicals, all the chlorinated methanes are produced together when the CH.sub.4 /Cl.sub.2 ratio is equimolar: ##STR1## Specific higher chlorination can be achieved by recycling the less highly chlorinated products.
The chlorinating scission of propene at temperatures of 600.degree. to 700.degree. C. and a pressure of 2 to 5 bar produces both perchloroethylene and carbon tetrachloride: EQU CH.sub.2 .dbd.CHCH.sub.3 +Cl.sub.2 .fwdarw.Cl.sub.2 C.dbd.CCl.sub.2 +CCl.sub.4 +HCl
Depending on the reaction conditions and the educt ratio, the quantity ratio CCl.sub.4 /Per can vary between 65/35 and 35/65, i.e. appreciable amounts of carbon tetrachloride are obtained in all cases.
As the principal use sector for CCl.sub.4, namely as an educt in the manufacture of chlorofluorohydrocarbons, will decline in the future because of the massive restrictions on production and the anticipated cessation of production, it was necessary to discover new possible uses for the CCl.sub.4 obtained in the afore-mentioned processes.
One possibility is to use CCl.sub.4 as an educt for the manufacture of chloromethanes, for example chloroform. Of the chloromethanes, chloroform in particular is currently important as an educt for the manufacture of polytetrafluoroethylene. In the synthesis of the tetrafluoroethylene monomer starting from chloroform, two chlorines are replaced with fluorine in the first step: EQU CHCl.sub.3 +2HF.fwdarw.CHClF.sub.2 +2HCl
The subsequent thermal dehydrochlorination of CHClF.sub.2 produces monomeric tetrafluoroethylene: EQU 2 CHClF.sub.2 .fwdarw.F.sub.2 C.dbd.CF.sub.2 +2 HCl
The object was therefore to develop a process for the manufacture of chloromethanes, especially chloroform, which starts from more highly chlorinated chloromethanes, for instance carbon tetrachloride, as the educt.